Correlation of molecular organization and substrate wettability in the self-assembly of n-alkylsiloxane monolayers

Atul N. Parikh, Bo Liedberg, Sundar V. Atre, Moses Ho, David L. Allara

Research output: Contribution to journalArticle

93 Citations (Scopus)

Abstract

Monolayers of n-octadecylsiloxane (CH3(CH2)17SiOxHy; ODS) were self-assembled from n-octadecyltrichlorosilane solutions onto a series of OH- and CH3-containing surfaces prepared from the self-assembly of controlled composition mixtures of HO(CH2)16SH and H3C(CH2)15SH on gold (RS/Au). Using null ellipsometry, infrared spectroscopy, and hexadecane contact angles; the coverages, chain structures, and surface wetting of the formed ODS assemblies were determined as a function of the OH fraction, fOH ≡ [OH]/[CH3 + OH], in the starting RS assembly. Three distinct ODS adsorption regimes were observed: (1) on pure CH3 surfaces no stable adsorbed layer forms; (2) for 0.1 ≲ fOH ≲ 0.8, the coverage is incomplete and monotonically increases with fOH and the ODS structures consist of a range of coexisting domains of nearly all-trans chains and disordered, liquid-like components with maximum disorder content, estimated as >80%, arising near fOH ∼ 0.5; and (3) for fOH > 0.8, a high coverage, close-packed monolayer is formed with predominantly all-trans chains tilted at 8-12° from the surface normal, a distinctly different tilt than the known value of 26-30° for the RS underlayer and an indication of strong structural decoupling (incommensurability) between the two highly organized layers. The fOH-dependence of the structures is explained on the basis of a previously proposed hypothesis that a continuous preadsorbed, substrate-bound water film is required for achieving maximum organization during n-alkylsiloxane self-assembly and that, in the present case of the OH/CH3 surfaces, the required water film structure at the preparation solution/substrate interface is not reached until high fOH values.

Original languageEnglish (US)
Pages (from-to)9996-10008
Number of pages13
JournalJournal of Physical Chemistry
Volume99
Issue number24
StatePublished - 1995
Externally publishedYes

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wettability
Self assembly
Wetting
self assembly
Monolayers
Substrates
Water
Ellipsometry
Gold
decoupling
assemblies
water
wetting
ellipsometry
Contact angle
Infrared spectroscopy
indication
assembly
infrared spectroscopy
disorders

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Parikh, A. N., Liedberg, B., Atre, S. V., Ho, M., & Allara, D. L. (1995). Correlation of molecular organization and substrate wettability in the self-assembly of n-alkylsiloxane monolayers. Journal of Physical Chemistry, 99(24), 9996-10008.

Correlation of molecular organization and substrate wettability in the self-assembly of n-alkylsiloxane monolayers. / Parikh, Atul N.; Liedberg, Bo; Atre, Sundar V.; Ho, Moses; Allara, David L.

In: Journal of Physical Chemistry, Vol. 99, No. 24, 1995, p. 9996-10008.

Research output: Contribution to journalArticle

Parikh, AN, Liedberg, B, Atre, SV, Ho, M & Allara, DL 1995, 'Correlation of molecular organization and substrate wettability in the self-assembly of n-alkylsiloxane monolayers', Journal of Physical Chemistry, vol. 99, no. 24, pp. 9996-10008.
Parikh, Atul N. ; Liedberg, Bo ; Atre, Sundar V. ; Ho, Moses ; Allara, David L. / Correlation of molecular organization and substrate wettability in the self-assembly of n-alkylsiloxane monolayers. In: Journal of Physical Chemistry. 1995 ; Vol. 99, No. 24. pp. 9996-10008.
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AU - Allara, David L.

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AB - Monolayers of n-octadecylsiloxane (CH3(CH2)17SiOxHy; ODS) were self-assembled from n-octadecyltrichlorosilane solutions onto a series of OH- and CH3-containing surfaces prepared from the self-assembly of controlled composition mixtures of HO(CH2)16SH and H3C(CH2)15SH on gold (RS/Au). Using null ellipsometry, infrared spectroscopy, and hexadecane contact angles; the coverages, chain structures, and surface wetting of the formed ODS assemblies were determined as a function of the OH fraction, fOH ≡ [OH]/[CH3 + OH], in the starting RS assembly. Three distinct ODS adsorption regimes were observed: (1) on pure CH3 surfaces no stable adsorbed layer forms; (2) for 0.1 ≲ fOH ≲ 0.8, the coverage is incomplete and monotonically increases with fOH and the ODS structures consist of a range of coexisting domains of nearly all-trans chains and disordered, liquid-like components with maximum disorder content, estimated as >80%, arising near fOH ∼ 0.5; and (3) for fOH > 0.8, a high coverage, close-packed monolayer is formed with predominantly all-trans chains tilted at 8-12° from the surface normal, a distinctly different tilt than the known value of 26-30° for the RS underlayer and an indication of strong structural decoupling (incommensurability) between the two highly organized layers. The fOH-dependence of the structures is explained on the basis of a previously proposed hypothesis that a continuous preadsorbed, substrate-bound water film is required for achieving maximum organization during n-alkylsiloxane self-assembly and that, in the present case of the OH/CH3 surfaces, the required water film structure at the preparation solution/substrate interface is not reached until high fOH values.

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